1. Field of the Invention
The present invention relates to a cylindrical lithium ion secondary battery, and more particularly, to a cylindrical lithium secondary battery having improved structural stability.
2. Description of Related Art
Recently, a large number of compact and lightweight electronic apparatus such as cellular phones, notebook computers, and camcorders have been developed and manufactured. These portable electronic apparatus are provided with battery packs so that the apparatus can operate without separate power supplies. Each battery pack includes at least one battery to drive the portable electronic apparatuses for a predetermined time.
Recently, battery packs have employed secondary (rechargeable) rechargeable batteries. Representative secondary batteries include nickel cadmium (Ni—Cd) batteries, nickel hydride (Ni-MH) batteries, and lithium rechargeable batteries such as lithium (Li) polymer batteries and Lithium ion (Li-ion) batteries.
Lithium secondary batteries have an operating voltage of 3.6V which is three times higher than a Ni—Cd battery or a Ni-MH battery. In addition, lithium secondary batteries have a high energy density per unit weight. Therefore, demand for lithium secondary batteries has rapidly increased.
For lithium secondary batteries, a lithium-based oxide is used as a positive electrode active material and carbon is used as a negative electrode active material. In general, batteries are classified as either liquid electrolyte batteries or polymer electrolyte batteries according to the type of the electrolyte used. Lithium secondary batteries using liquid electrolyte are called lithium ion batteries, and lithium secondary batteries using polymer electrolyte are called lithium polymer batteries. Lithium secondary batteries are manufactured in various shapes and are classified, for example, into cylindrical batteries, prismatic batteries, and pouch-type batteries.
In general, a lithium secondary battery includes an electrode assembly, a case adapted to receive the electrode assembly, and electrolyte solution injected inside the case to enable the lithium ions to move. The electrode assembly includes a positive electrode plate coated with a positive electrode active material, a negative electrode plate coated with a negative electrode active material, and a separator interposed between the positive and negative electrode plates to prevent a short circuit between the two electrode plates and to allow only lithium ions to pass through.
The electrode assembly of a cylindrical lithium secondary battery is constructed by overlaying and rolling the positive electrode plate connected to a positive tab, the negative electrode plate connected to a negative tab, and the separator.
The electrode assembly is then inserted into the case and fixed therein. The electrolyte solution is injected into the case and the opening of the case is sealed with a cap assembly. Typically, insulation plates are combined with the electrode assembly, and the negative tab of the electrode assembly is attached to the cylindrical case.
The number of cylindrical lithium secondary batteries having high capacity is rapidly increasing. Accordingly, the positive and negative tabs of the electrode assembly have become thicker.
However, in a cylindrical lithium secondary battery, if the negative tab is thick, an insulation plate attached to the negative tab may become deformed so as to create a spatial non-uniformity in the battery. The non-uniformity may adversely affect the safety of the cylindrical lithium secondary battery when an external pressure is applied to the battery.